Book entitled: Feedforward Linear Power Amplifiers. Nick Pothecary. Artech House, 1999. Pages 124-192
This invention relates to a multi-tone, high frequency, and high power linearizer amplifier with low intermodulation distortion. Similar type of amplifiers uses predistortion or feedforward circuits or a combination of both. These amplifiers are typically used in wireless telecommunications applications and can be used in other applications requiring a high level of linearity including fiber optic and military systems.
When amplifiers are used at their high power range of their capability they become non-linear. Non-linear operation results in distortion of the input signal at the output of an amplifier. Distortion products that fall in adjacent active channels cause unwanted interference. New wireless communication signals have high peak-to-average ratio causing amplifiers to temporarily operate in their non-linear region and cause distortion affecting adjacent communication channels. Predistortion and feedforward amplifiers are used to xe2x80x9clinearizexe2x80x9d or reduce the distortions from amplifiers used in applications requiring closely spaced channels at high power levels.
The prior art techniques like predistortion uses independently created distortion products ahead of the amplifier. Feedforward uses a sample of the output signal from the non-linear device to achieve a means of linearization of the distortion products.
Predistortion Amplifiers
The basic concept shown in FIG. 2 is to create a circuit that has output signal characteristics that are inversely proportional to the amplifier that it drives, so that at the point that the main amplifier transfer curve deviates from linear, the predistorter driver transfer curve is opposite, to maintain the desired output linearity. Also, the predistorter drives the amplifier so that the intermodulation products are canceled at its output by generating similar 3rd order intermodulation products as those created by the amplifier but in anti-phase.
Because of the difficulty in creating the right transfer characteristic over all input conditions a predistorter only provides cancellation of 3rd order products and are limited to the narrow maximum input level range of the amplifier. Predistorters also reduce the gain of the overall amplifier by the amount of loss of the predistorter circuit. Adaptive predistorter techniques are also used to compensate for aging and temperature, using a sample the output signal to adjust the pre-distorting transfer characteristics over temperature time etc. These use memory-based circuitry or base band frequency control and are usually narrower band and much more complex than non-adaptive types.
Feedforward Amplifiers
The basic concept shown in FIG. 3 is to create a stable, delayed flow of the signals to improve the linearity of the main amplifier. Two loops are used. The first loop generates the error signal consisting ideally of the distortion products and the second uses the error signal to insert it back into the output signal flow and cancel the distortion out of the main amplifier.
First Loop: The input signal is split at the input coupler C1. One path goes to the main amplifier and the other is delayed and fed to a summing network at coupler C4. The output signal of the amplifier is sampled at the output of the Main Amplifier on coupler C2, and fed to the summing network at coupler C4. By proper delay and level control, the fundamental input signal is suppressed and the distortion signals remain. This left over signal called xe2x80x9cerror signalxe2x80x9d is then used in the second loop.
Second Loop: The error and fundamental residue signal from the first loop is amplified and feeds into the coupled port of the output coupler C3 with the intermod products at a level and phase that causes the Main Amplifier""s output non-linearity to cancel. The output of the main amplifier containing the amplified input signal and the non-linearity is delayed to meet the error-correcting signal at the output coupler C3 at the exact time to form a continuous signal flow without feedback.
In actual practice is impossible to maintain a perfect balance over a range of frequencies and operating conditions such as input drive, temperature, and tolerances of components. The loops have to be balanced to a very high degree of precision (normally  less than 0.5 dB amplitude and  less than 5 degree of phase) required for high distortion cancellation. When the feedforward amplifier is not balanced, some of the fundamental output signal will not cancel completely and feeds thru to the second loop of the feedforward linearizer. This signal is amplified by the error amplifier and gets back into the main output at C4. Depending on the phase and level it adds or subtract from the main signal. If the level is high enough to drive the Error Amplifier into its non-linear condition, the intermodulation products of the Error Amplifier feeds into the output at C3 causing the system to become non-linear again. The error amplifier, therefore, has to be properly sized to be capable of handling these unwanted signals so as to maintain the desired level of linearity over the full frequency range and input levels. This problem results in a lower level of efficiency.
A significant deficiency of the feedforward configuration occurs when the Main Amplifier is used above the point where gain compression starts. At this point the first loop becomes un-balanced. The un-balance causes the level of the error signal to increase significantly driving the error amplifier to become non-linear and feed new distortion back into the output.
U.S. Pat. No. 6,111,462, and the simpler European Patent Application EP 0 762 630 A1 describes feedforward configurations that provide both cancellation and higher power by using xe2x80x9cparallel main power amplifiersxe2x80x9d and a properly delayed sample of the output intermodulation products and the reference fundamental input signal. The attenuator connected to coupler C2 can replaced with a phase shifter. It creates an error with the proper phase and amplitude so that it can be amplified with a high power amplifier in the error path and then combined with the proper amplitude and phase at the output to achieve higher power with cancellation. See FIG. 3. These methods improve on the efficiency disadvantages of the classical feedforward linearizer. The referenced patents also have similar difficulties as the standard configuration when the signal levels crossover to the gain compression region of the Main amplifier thereby causing 1st loop imbalance and injection of intermodulation products into the output.
The use of predistortion with feedforward techniques is needed to operate above the gain compression point area of the main amplifier but at a much added circuit complexity.
In accordance with the present invention a linearizing method that uses of a fraction of the reflected input signal of an amplifier by means of a distortion-correcting path for the purpose of reducing the output distortion products created by said amplifier. The delayed output signal of said amplifier and the output signal of said distortion correcting path are combined to provide significant distortion reduction and increased fundamental power at the output port of said combiner.
The Reflect-Forward Adaptive Linearizer Amplifier (RFAL Amplifier) invention has the following improvements over prior art:
a. The xe2x80x9cRFALxe2x80x9d Amplifier provides substantial linear operation improvement when operating with power levels up to the 1 dB gain compression point of the Main Amplifier in FIG. 1.
b. Input overloading characteristics are significantly better than those encountered in FIG. 3 feedforward configurations. Also provides higher peak-to-average power ratio operation.
c. The RFAL Amplifier uses a lower number of couplers and shorter delay lines resulting in lower circuit losses and simplicity. The improvement in operational linearity at a higher power level allows the use of lower power output capability, and more economical semiconductors to attain the same level of performance that prior art configurations provide.
d. The distortion-correcting path can be aligned independently from the fundamental path both in amplitude and phase. No output error signal or output sampling coupler is used. Results in lower losses and makes it easier to align for optimum distortion cancellation, output power, and gain flatness. The RFAL Amplifier uses three basic independently adjusted paths that begin at the input of the assembly and combine at the output of the assembly. These paths are:
A) Main 1 Amplifier path.
B) Reflected Distortion-Cancellation path
C) Forward path.
(Paths C and B travel through the same Booster and Main 2 Amplifier.)
e. Three identical amplifiers can be used to double the output power of the Main Amplifier with significant intermodulation distortion cancellation features and provides improved gain and temperature tracking. Also simpler manufacture, temperature and aging compensation can be realized by using the same amplifiers.
f. The signals all flow in a forward direction, with similar arrival time at output, achieving the cancellation of intermod products with maximum output power.
g. The RFAL Amplifier provides real-time adaptive linearization performance with any number of signals and modulation schemes.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.